The Caspian Sea presently lies about 28 m (92 ft) below sea level in the Caspian Depression, to the east of the Caucasus Mountains and to the west of the vast steppe of Central Asia. The sea bed in the southern part reaches as low as 1,023 m (3,356 ft) below sea level, which is the second lowest natural depression on earth after Lake Baikal (−1,180 m, −3,871 ft). The ancient inhabitants of its coast perceived the Caspian Sea as an ocean, probably because of its saltiness and large size.
The sea has a surface area of 371,000 km2 (143,200 sq mi) (not including the detached lagoon of Garabogazköl) and a volume of 78,200 km3 (18,800 cu mi). It has a salinity of approximately 1.2% (12 g/l), about a third of the salinity of most seawater.
The word Caspian is derived from the name of the Caspi, an ancient people who lived to the southwest of the sea in Transcaucasia. Strabo wrote that "to the country of the Albanians belongs also the territory called Caspiane, which was named after the Caspian tribe, as was also the sea; but the tribe has now disappeared". Moreover, the Caspian Gates, which is the name of a region in Iran's Tehran province, possibly indicates that they migrated to the south of the sea. The Iranian city of Qazvin shares the root of its name with that of the sea. In fact, the traditional Arabic name for the sea itself is Baḥr al-Qazwin (Sea of Qazvin).
In classical antiquity among Greeks and Persians it was called the Hyrcanian Ocean. In Persian antiquity, as well as in modern Iran, it is known as درياى خزر, Daryā-e Khazar; it is also sometimes referred to as Mazandaran Sea (Persian: دریای مازندران) in Iran. Ancient Arabic sources refer to it as Baḥr Gīlān (بحر گیلان) meaning "the Gilan Sea".
Turkic languages refer to the lake as Khazar Sea. In Turkmen, the name is Hazar deňizi, in Azeri, it is Xəzər dənizi, and in modern Turkish, it is Hazar denizi. In all these cases, the second word simply means "sea", and the first word refers to the historical Khazars who had a large empire based to the north of the Caspian Sea between the 7th and 10th centuries. An exception is Kazakh, where it is called Каспий теңізі, Kaspiy teñizi (Caspian Sea).